Plural polygon source pattern for MOSFET

ABSTRACT

A high power MOSFET has a plurality of closely packed polygonal sources spaced from one another on one surface of a semiconductor body. An elongated gate electrode is exposed in the spacing between the polygonal sources and cooperates with two channels, one for each adjacent source electrode, to control conduction from the source electrode through the channel and them to a drain electrode on the opposite surface of the semiconductor body. The conductive region adjacent the channel and between adjacent sources is relatively highly conductive in the section of the channel adjacent to the surface containing the sources. The polygonal shaped source members are preferably hexagonal so that the distances between adjacent sources is relatively constant throughout the device. Each polygonal region has a relatively deep central portion and a shallow outer shelf portion. The shelf portion generally underlies an annular source region. The deep central portion underlies an aluminum conductive electrode and is sufficiently deep that it will not be fully penetrated by aluminum spiking.

This application is a continuation of application Ser. No. 243,544,filed Mar. 13, 1981, now abandoned, which was a continuation ofapplication Ser. No. 038,662, filed May 14, 1979, now abandoned.

RELATED APPLICATIONS

This application is related to application Ser. No. 951,310, filed Oct.13, 1978, now abandoned and refiled as continuation application Ser. No.232,713, filed Feb. 9, 1981, now U.S. Pat. No. 4,376,286, entitled HIGHPOWER MOSFET WITH LOW ON-RESISTANCE AND HIGH BREAKDOWN VOLTAGE, in thenames of Alexander Lidow and Thomas Herman, and assigned to the assigneeof the present invention.

BACKGROUND OF THE INVENTION

This invention relates to MOSFET devices, and more specifically relatesto a novel source pattern for a MOSFET device of the type disclosed inabove-mentioned U.S. Pat. No. 4,376,286 wherein a plurality ofpolygonal-shaped source elements are disposed over the surface of asemiconductor body and are spaced from one another by a closelycontrolled dimension.

High power MOSFETS having low on-resistance and high breakdown voltageare known and are shown in the above-noted U.S. Pat. No. 4,376,286. Inthe above application, the source electrodes are spaced, interdigitatedsource regions spaced from one another by two parallel channel regionscovered by a common gate. The device has exceptionally low on-resistancealong with the usual advantages of the MOSFET device over the bi-polardevice particularly by virtue of a relatively high conductivity regiondisposed between the two adjacent channels and leading to a common drainelectrode.

It has been found that an interdigitated structure has a relatively lowpacking density. Moreover, the interdigitated arrangement disclosed inthe above U.S. Pat. No. 4,376,286 requires relatively complicated masksand has a relatively high capacitance.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a novel high power MOSFET device with lowforward resistance where a very high packing density is available andwhich can be made with relatively simple masks. The device further hasrelatively low capacitance. Typically, the device may be made throughthe use of phosphorus implantation and D-MOS fabrication techniques butany desired technique can be used.

Each of the individual spaced source regions, in accordance with theinvention, is polygonal in configuration and is preferably hexagonal toensure a constant spacing along the major lengths of the sourcesdisposed over the surface of the body. An extremely large number ofsmall hexagonal source elements may be formed in the same surface of thesemiconductor body for a given device. By way of example, 6,600hexagonal source regions can be formed in a chip area having a dimensionof about 100 by 140 mils to produce an effective channel width of about22,000 mils, thus permitting very high current capacity for the device.

The space between the adjacent sources may contain a polysilicon gate orany other gate structure where the gate structure is contacted over thesurface of the device by elongated gate contact fingers which ensuregood contact over the full surface of the device.

Each of the polygonal source regions is contacted by a uniformconductive layer which engages the individual polygonal sources throughopenings in an insulation layer covering the source regions, whichopenings can be formed by conventional D-MOS photolithographictechniques. A source pad connection region is then provided for thesource conductor and a gate pad connection region is provided for theelongated gate fingers and a drain connection region is made to thereverse surface of the semiconductor device.

A plurality of such devices can be formed from a single semiconductorwafer and the individual elements can be separated from one another byscribing or any other suitable method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a completed element on a semiconductor waferprior to the separation of the element away from the remainder of thewafer.

FIG. 2 is an enlarged detail of the gate pad to illustrate therelationship of the gate contact and the source polygons in the regionof the gate pad.

FIG. 3 is a detailed plan view of a small portion of the source regionduring one stage of the manufacturing process of the device.

FIG. 4 is a cross-sectional view of FIG. 3 taken across the section line4--4 in FIG. 3.

FIG. 5 is similar to FIG. 4 and shows the addition of a polysilicongate, a source electrode means and drain electrode to the wafer.

DETAILED DESCRIPTION OF THE DRAWINGS

The polygon configuration of the source regions of the present inventionis best shown in FIGS. 3, 4 and 5 which are first described.

Referring first to FIGS. 3 and 4, the device is shown prior to theapplication of the gate, source and drain electrodes. The manufacturingprocess can be of any desired type. The manufacturing process describedin U.S. Pat. No. 4,376,286, referred to above, which is incorporatedherein by reference, can be used whereby D-MOS fabrication techniquesand ion implantation techniques can be advantageously employed for theformation of the junction and placement of the electrode in the mostadvantageous way.

The device is described as an N channel enhancement type device. It willbe apparent that the invention will also apply to P channel devices andto depletion mode devices.

The device of FIGS. 3 and 4 has a plurality of polygonal source regionson one surface of the device, where these polygonal regions arepreferably hexagonal in shape. Other shapes such as squares could havebeen used but the hexagonal shape provides better uniformity of spacingbetween adjacent source region perimeters.

In FIGS. 3 and 4, the hexagonal source regions are formed in a basicsemiconductor body or wafer which can be an N type wafer 20 ofmonocrystalline silicon which has a thin N-epitaxial region 21 depositedthereon as best shown in FIG. 4. All junctions are formed in epitaxialregion 21. By using suitable masks, a plurality of P type base regionssuch as regions 22 and 23 in FIGS. 3 and 4 are formed in one surface ofthe semiconductor wafer region 21, where these regions are generallypolygonal in configuration and, preferably, are hexagonal.

A very large number of such polygonal regions are formed. For example,in a device having a surface dimension of 100 by 140 mils, approximately6600 polygonal regions are formed to produce a total channel width ofabout 22,000 mils. Each of the polygonal regions may have a widthmeasured perpendicular to two opposing sides of the polygon of about 1mil or less. The regions are spaced from one another by a distance ofabout 0.6 mil when measured perpendicularly between the adjacentstraight sides of adjacent polygonal regions.

The P+ regions 22 and 23 will have a depth d which is preferably about 5microns to produce a high and reliable field characteristic. Each of theP regions has an outer shelf region shown as shelf regions 24 and 25 forP regions 22 and 23, respectively, having a depth s of about 1.5microns. This depth should be as small as possible to reduce thecapacitance of the device.

Each of the polygon regions including polygonal regions 22 and 23receive N+ polygonal ring regions 26 and 27, respectively. Shelves 24and 25 are located beneath regions 26 and 27, respectively. N+ regions26 and 27 cooperate with a relatively conductive N+ region 28 which isthe N+ region disposed between adjacent P type polygons to define thevarious channels between the source regions and a drain contact whichwill be later described.

The highly conductive N+ regions 28 are formed in the manner describedin U.S. Pat. No. 4,376,286, referred to above, and are the subject ofthat application and produce a very low forward resistance for thedevice.

In FIGS. 3 and 4, it will be noted that the entire surface of the waferis covered with an oxide layer or combined conventional oxide andnitride layers which are produced for the formation of the variousjunctions. This layer is shown as the insulation layer 30. Theinsulation layer 30 is provided with polygonal shaped openings such asopenings 31 and 32 immediately above polygonal regions 22 and 23.Openings 31 and 32 have boundaries overlying the N+ type source rings 26and 27 for the regions 22 and 23, respectively. The oxide strips 30,which remain after the formation of the polygonal shaped openings,define the gate oxide for the device.

Electrodes may then be applied to the device as shown in FIG. 5. Theseinclude a polysilicon grid which includes polysilicon sections 40, 41and 42 which overlie the oxide sections 30.

A silicon dioxide coating is then deposited atop the polysilicon grid 40shown as coating sections 45, 46 and 47 in FIG. 5 which insulates thepolysilicon control electrode and the source electrode which issubsequently deposited over the entire upper surface of the wafer. InFIG. 5 the source electrode is shown as conductive coating 50 which maybe of any desired material, such as aluminum. A drain electrode 51 isalso applied to the device.

The resulting device of FIG. 5 is an N channel type device whereinchannel regions are formed between each of the individual sources andthe body of the semiconductor material which ultimately leads to thedrain electrode 51. Thus, a channel region 60 is formed between thesource ring 26, which is connected to source electrode 50, and the N+region 28 which ultimately leads to the drain electrode 51. Channel 60is inverted to N type conductivity upon the application of a suitablecontrol voltage to the gate 40. In a similar manner, channels 61 and 62are formed between the source region 26, which is connected to theconductor 50, and the surrounding N+ region 28 which leads to the drain51. Thus, upon application of a suitable control voltage to thepolysilicon gate (including finger 41 in FIG. 5), channels 61 and 62become conductive to permit majority carrier conduction from the sourceelectrode 50 to the drain 51. Note that channels labeled 60 and 61 ofFIG. 5 are the same annular channel which is formed within the region22. Similarly, channels 62 and 63 are the same annular channel formed inthe region 23.

Each of the sources form parallel conduction paths where, for example,channels 63 and 64 beneath gate element 42 permit conduction from thesource ring 27 and an N type source strip 70 to the N+ region 28 andthen to the drain electrode 51.

It is to be noted that FIGS. 4 and 5 illustrate an end P type region 71which encloses the edge of the wafer.

The contact 50 of FIG. 5 is preferably an aluminum contact. It will benoted that the contact region for the contact 50 lies entirely over andin alignment with the deeper portion of the P type region 22. This isdone since it was found that aluminum used for the electrode 50 mightspike through very thin regions of the P type material. Thus, onefeature of the present invention is to ensure that the contact 50 liesprincipally over the deeper portions of the P regions such as P regions22 and 23. This then permits the active channel regions defined by theannular shelves 24 and 25 to be as thin as desired in order tosubstantially reduce the device capacitance.

FIG. 1 illustrates one completed device using the polygonal sourcepattern of FIG. 5. The completed device shown in FIG. 1 is containedwithin the scribe regions 80, 81, 82 and 83 which enable the breakingout of a plurality of unitary devices each having a dimension of 100 by140 mils from the body of the wafer.

The polygonal regions described are contained in a plurality of columnsand rows. By way of example, the dimension A contains 65 columns ofpolygonal regions and may be about 83 mils. The dimension B may contain100 rows of polygonal regions and may be about 148 mils. Dimension C,which is disposed between a source connection pad 90 and a gateconnection pad 91, may contain 82 rows of polygonal elements.

The source pad 90 is a relatively heavy metal section which is directlyconnected to the aluminum source electrode 50 and permits convenientlead connection for the source.

The gate connection pad 91 is electrically connected to a plurality ofextending fingers 92, 93, 94 and 95 which extend symmetrically over theouter surface of the area containing the polygonal regions and makeelectrical connection to the polysilicon gate as will be described inconnection with FIG. 2.

Finally the outer circumference of the device contains the P+ (shown as"P" in the drawings) deep diffusion ring 71 which may be connected to afield plate 101 shown in FIG. 1.

FIG. 2 shows a portion of the gate pad 91 and the gate fingers 94 and95. It is desirable to make a plurality of contacts to the polysilicongate in order to reduce the R-C delay constant of the device. Thepolysilicon gate has a plurality of regions including regions 110, 111,112 and the like which extend outwardly and receive extensions of thegate pad and the gate pad elements 94 and 95. The polysilicon gateregions may be left exposed during the formation of the oxide coating45-46-47 in FIG. 5 and are not coated by the source electrode 50. Notethat in FIG. 2 the axis 120 is the axis of symmetry 120 which is thatshown in FIG. 1.

Although the present invention has been described in connection with apreferred embodiment thereof, many variations and modifications will nowbecome apparent to those skilled in the art. It is preferred, therefore,that the present invention be limited not by the specific disclosureherein, but only by the appended claims.

What is claimed is:
 1. A high power MOSFET device having more than 1000parallel-connected individual FET devices closely packed into arelatively small area comprising:a thin wafer of semiconductor materialhaving first and second spaced, parallel planar surfaces; at least afirst portion of the thickness of said wafer which extends from saidfirst planar surface consisting of an epitaxially deposited region of afirst conductivity type; a plurality of symmetrically disposed laterallydistributed hexagonal base regions each having a second conductivitytype formed in said epitaxially deposited region and extending for agiven depth beneath said first planar surface; said hexagonal baseregions spaced at said first surface from surrounding ones by asymmetric hexagonal lattice of semiconductor material of said firstconductivity type; each side of each of said hexagonal base regionsbeing parallel to an adjacent side of another of said hexagonal baseregions; a hexagonal annular source region of said first conductivitytype formed in an outer peripheral region of each of said hexagonal baseregions and extending downwardly from said first planar surface to adepth less than the depth of said base regions; an outer rim of each ofsaid annular source regions being radially inwardly spaced from an outerperiphery of its respective hexagonal base region to form an annularchannel between each of said outer rims of said annular source regionsand said symmetric hexagonal lattice of semiconductor material of saidfirst portion of said wafer; a common source electrode formed on saidfirst planar surface and connected to a plurality of said annular sourceregions and to interiorly adjacent surface areas of their saidrespective hexagonal base regions; a drain electrode connected to saidsecond planar surface of said wafer; an insulation layer means on saidfirst planar surface and overlying at least said annular channels; and apolysilicon gate electrode atop said insulation layer means and operableto invert said annular channels.
 2. The MOSFET device of claim 1,wherein the material of said lattice of said first conductivity type hasa relatively high impurity concentration region compared to theconcentration of the remainder of said first portion of said wafer; saidrelatively high concentration region having a depth greater than thedepth of said source regions and less than the depths of said pluralityof base regions.
 3. A high power MOSFET device having more than 1000parallel-connected individual FET devices closely packed into arelatively small area comprising:a thin wafer of semiconductor materialhaving first and second spaced, parallel planar surfaces; at least afirst portion of the thickness of said wafer which extends from saidfirst planar surface consisting of an epitaxially deposited region of afirst conductivity type; a plurality of symmetrically disposed laterallydistributed polygonal base regions each having a second conductivitytype formed in said lightly doped region and extending for given depthbeneath said first planar semiconductor surface; said polygonal baseregions spaced at said first surface from surrounding ones by asymmetric polygonal lattice of semiconductor material of said firstconductivity type; each side of each of said polygonal base regionsbeing parallel to an adjacent side of another of said polygonal baseregions; a polygonal annular source region of said first conductivitytype formed in an outer peripheral region of each of said polygonal baseregions and extending downwardly from said first planar surface to adepth less than the depth of said base regions; an outer rim of each ofsaid annular source regions being radially inwardly spaced from an outerperiphery of its respective polygonal base region to form an annularchannel between each of said outer rims of said annular source regionsand said symmetric polygonal lattice of semiconductor material of saidfirst portion of said wafer; a common source electrode formed on saidfirst planar surface and connected to a plurality of said annular sourceregions and to interiorly adjacent surface areas of their saidrespective polygonal base regions; a drain electrode connected to saidsecond planar semiconductor surface of said wafer; an insulation layermeans on said first planar surface and overlying at least said annularchannels; and a polysilicon gate electrode atop said insulation layermeans and operable to invert said annular channels.
 4. The MOSFET deviceof claim 1 or 3, wherein each of said base regions occupies a totallateral area having a diameter less than about 1 mil and are spaced fromadjacent base regions by about 0.6 mil.
 5. The device of claim 1 or 3,wherein said 40M, is an N type conductivity, said base regions are a Ptype conductivity and said source regions are an N type conductivity. 6.The MOSFET device of claim 3, wherein the material of said lattice ofsaid first conductivity type has a relatively high impurityconcentration region compared to the concentration of the remainder ofsaid first portion of said wafer; said relatively high concentrationregion having a depth greater than the depth of said source regions andless than the depths of said plurality of base regions.
 7. A verticalconduction high power MOSFET device exhibiting relatively lowon-resistance and relatively high breakdown voltage; said devicecomprising:a wafer of semiconductor material having planar first andsecond opposing semiconductor surfaces; said wafer of semiconductormaterial having a relatively lightly doped major body portion forreceiving junctions and being doped with impurities of a firstconductivity type; a plurality of highly packed, equally spacedsymmetrically disposed identical polygonal base regions of a secondconductivity type formed in said wafer, each extending from said firstplanar semiconductor surface to a first depth beneath said first planarsemiconductor surface; said polygonal base regions spaced fromsurrounding ones by a symmetric polygonal lattice of semiconductormaterial of said first conductivity type; the space between adjacentones of said polygonal base regions defining a common conduction regionof said first conductivity type extending downwardly from said firstplanar semiconductor surface; a respective polygonal annular sourceregion of said first conductivity type formed within each of saidpolygonal base regions and extending downwardly from said first planarsemiconductor surface to a depth less than said first depth; each ofsaid polygonal annular source regions being laterally spaced along saidfirst planar semiconductor surface from the facing respective edges ofsaid common conduction region thereby to define respective coplanarannular channel regions along said first planar semiconductor surfacebetween the polygonal sides of each of said polygonal annular sourceregions and said common conduction region; a common source electrodemeans connected to said polygonal annular source regions and theirrespective base regions; gate insulation layer means on said firstplanar semiconductor surface, disposed at least on said coplanar channelregions; gate electrode means on said gate insulation layer means andoverlying said coplanar channel regions; a drain conductive regionremote from said common conduction region and separated therefrom bysaid relatively lightly doped major body portion and extending to saidsecond semiconductor surface; and a drain electrode coupled to saiddrain conductive region.
 8. A high power MOSFET device exhibitingrelatively low on-resistance and relatively high breakdown voltage; saiddevice comprising:a wafer of semiconductor material having planar firstand second opposing semiconductor surfaces; said wafer of semiconductormaterial having a relatively lightly doped major body portion forreceiving junctions and being doped with impurities of a firstconductivity type; at least first and second spaced base regions of asecond conductivity type formed in said wafer and extending downwardlyfrom said first planar semiconductor surface to a first depth beneathsaid first planar semiconductor surface; the space between said at leastfirst and second spaced base regions defining a common conduction regionof a first conductivity type at a given first planar semiconductorsurface location; said common conduction region extending downwardlyfrom said first planar semiconductor surface; first and second annularsource regions of said first conductivity type formed in said first andsecond spaced base regions respectively at said first planarsemiconductor surface locations to a depth less than said first depth;said first and second annular source regions being laterally spacedalong said first planar semiconductor surface from the facing respectiveedges of said common conduction region thereby to define first andsecond channel regions along said first planar semiconductor surfacebetween each pair of said first and second annular source regions,respectively, and said common conduction region; each of said first andsecond channel regions being coplanar with one another; a common sourceelectrode means connected to said first and second annular sourceregions and their respective first and second base regions; gateinsulation layer means on said first planar semiconductor surface,disposed at least on said first and second channel regions; gateelectrode means on said gate insulation layer means and overlying saidfirst and second channel regions; a drain conductive region remote fromsaid common conduction region and separated therefrom by said relativelylightly doped major body portion and extending to said secondsemiconductor surface; a drain electrode coupled to said drainconductive region; and each of said at least first and second spacedbase regions having a polygonal configuration; each of said first andsecond annular source regions having a polygonal configurationconforming to that of their respective base region.
 9. The MOSFET deviceof claims 7 or 8 wherein said common conduction region is relativelyhighly doped, compared to said relatively lightly doped major bodyportion and extends downwardly from said first planar semiconductorsurface to a depth greater than the depth of said source regions,whereby resistance to current flow at the boundaries between saidchannel regions and said common conduction region and between saidcommon conduction region and said relatively lightly doped major bodyportion is reduced without reducing breakdown voltage.
 10. The MOSFETdevice of claim 9, wherein the depth of said common conduction region isless than said first depth of said base regions.
 11. The MOSFET deviceof claim 1, 3, 35 or 36, wherein each of said base regions has adeepened central region and a shallower outer peripheral region, saiddeepened central region extending laterally under only a portion of itsassociated source region.
 12. The MOSFET device of claim 11, whereinsaid deepened central region has a depth of approximately 5 microns andsaid shallower outer peripheral region has a depth of greater thanapproximately 1.5 microns.
 13. The device of claim 32, 35 or 36, whereinthere are in excess of about 1000 source regions each having a width ofabout 1 mil.
 14. The device of claim 7 or 8, wherein said lightly dopedwafer portion is an N type conductivity, said base regions are a P typeconductivity and said source regions are an N type conductivity.